Add gn args and BUILDFLAGS for OfflinePages, instead on relying on is_android and is_official_build.

components/offline_pages/features/BUILD.gn

+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/buildflag_header.gni")
+import("//components/offline_pages/features/features.gni")
+
+# This file is in a separate directory so all targets in the build can refer to
+# the buildflag header to get the necessary preprocessor defines without
+# bringing in all of offline_pages. Other targets can depend on this target
+# regardless of whether extensions are enabled.
+
+buildflag_header("features") {
+  header = "features.h"
+  flags = [
+    "ENABLE_OFFLINE_PAGES=$enable_offline_pages",
+    "ENABLE_OFFLINE_PAGES_HARNESS=$enable_offline_pages_harness",
+  ]
+}